System and method for magnetic position tracking

ABSTRACT

An improved system for magnetic position tracking of a device includes a magnetic transmitter, a magnetic sensor, a computing system and a polarity inverter. The magnetic transmitter includes at least one transmitter coil that outputs a transmitted magnetic field having a time derivative component. The magnetic sensor includes at least one sensor coil that has coil terminals having a polarity, and the sensor coil is responsive to the time derivative component of the transmitted magnetic field and outputs a sensor signal. The computing system computes position and angular orientation data of a device based on the sensor signal and the polarity inverter is configured to connect to the coil terminals and to cause the polarity of the coil terminals to be reversed according to a switching signal.

FIELD OF THE INVENTION

The present invention relates to an improved system and method formagnetic position tracking, and more particularly to a system and amethod that reduces the magnetic field induced noise signal in thesensor interconnect system by periodically switching the polarity of thenoise signal.

BACKGROUND OF THE INVENTION

Magnetic position tracking systems are becoming more widely used in themedical field, particularly when paired with an ultrasound imagingsystem. Due to the problems introduced into magnetic systems byconductive metals, medical magnetic tracking systems may operate in alow frequency band, in the sub 2 KHz range down to near DC levels.Distortion of the transmitted fields due to nearby conductive metals isminimized when operating in this low frequency range. A problem whicharises due to these low frequencies is that the magnetic signals tend tobe less affected by signal shielding materials such as aluminum orcopper which are effective at higher frequencies. The shields for lowfrequency must employ high permeability materials and the design must beoptimized such that leakage fields are well controlled. This makes thedesign of low frequency shielding much more difficult than for higherfrequencies where thin conductive foils and loosely fitting shells canbe employed. Due to the sensitive nature of the signals from themagnetic sensors, the signal path interconnect must be carefullydesigned to minimize sensitivity to the transmitted field. Electromotiveforce (EMF) errors are induced into the interconnect system if there isan unbalanced loop area within the interconnect system that is exposedto the transmitted field. In the case of an ultrasound probe, the probeinterconnect system is designed to accommodate hundreds of co-axialcable elements and their associated terminations. This type ofinterconnect presents a relatively large unbalanced loop area into thesignal path of the magnetic sensor.

Prior art systems have avoided this problem by running the optimizedmagnetic interconnect cable assembly adjacent to the probe interconnectcable assembly. The external mounting of the magnetic sensor and thebulk of a second independent cable running alongside the probe cable isobjectionable to many end users. In order to disconnect a probe from theultrasound chassis, both the probe interconnect and magnetic sensorinterconnect must be disconnected. The mass of the probe interconnect,which is attached to the magnetic sensor cable and connector, stressesthe smaller interconnect causing reliability concerns. Anotherlimitation of prior art systems is seen when the sensor signals must bepassed through a connector which shares the same physical structure as atherapeutic device, such as is found on an endoscope. In this case, themagnetic signal must be contained within the instrument due to sizeconstraints. Currently, prior art systems employ magnetic shieldingaround the magnetic portion of the instrument connector. This shieldingcan become bulky, complex, and expensive. Sterilization and reprocessingare needed in order to safely re-use such an instrument, and these costsare moving the industry towards inexpensive disposable devices. Theability to pass the magnetic sensor signals through a single,uncomplicated, low cost interconnect, without adding large cost elementsto the magnetic sensor, is thus very desirable.

SUMMARY OF THE INVENTION

In general, in one aspect, the invention features a system for magneticposition tracking of a device including a magnetic transmitter, amagnetic sensor, a computing system and a polarity inverter. Themagnetic transmitter includes at least one transmitter coil that outputsa transmitted magnetic field having a time derivative component. Themagnetic sensor includes at least one sensor coil that has coilterminals having a polarity, and the sensor coil is responsive to thetime derivative component of the transmitted magnetic field and outputsa sensor signal. The computing system computes position and angularorientation data of a device based on the sensor signal and the polarityinverter is configured to connect to the coil terminals and to cause thepolarity of the coil terminals to be reversed according to a switchingsignal.

Implementations of this aspect of the invention may include one or moreof the following features. The system may further include a signinverter configured to invert a digitized output from an analog todigital (A/D) converter. The sign inverter is operated concurrently withthe switching signal, so that the polarity of the coil terminals ismaintained at the computing system's input. The system may furtherinclude a synchronizer configured to operate concurrently with themagnetic transmitter. The system may further include averaging means.The sign inverter is also configured to invert the sensor signal at theA/D converter's input. The transmitted magnetic field may be a sinusoid.The sinusoid may include a plurality of sine waves. The sinusoid may becontinuous with respect to time. The sinusoid may be time divisionmultiplexed. The transmitted magnetic field may have one of trapezoidal,triangular, half sinusoid, exponential, or square amplitude versus timecharacteristics shape. The polarity inverter is located adjacent to themagnetic sensor. The polarity inverter is connected to the magneticsensor via a twisted pair cable. The polarity inverter may be an analogswitch. The switching signal is transmitted wirelessly or via a wiredconnection. The averaging means is configured to sum signals receivedwith opposite polarity from the sign inverter. The averaging means maybe a lowpass filter.

In general, in another aspect, the invention features a method formagnetic position tracking of a device including the following steps.Providing a magnetic transmitter having at least one transmitter coil.The transmitter coil outputs a transmitted magnetic field having a timederivative component. Providing a magnetic sensor having at least onesensor coil. The sensor coil has coil terminals having a polarity, andthe sensor coil is responsive to the time derivative component of thetransmitted magnetic field and outputs a sensor signal. Providing acomputing system for computing position and angular orientation data ofa device based on the sensor signal and providing a polarity inverterconfigured to connect to the coil terminals and to cause the polarity ofthe coil terminals to be reversed according to a switching signal.

This invention is applicable to electromagnetic tracking of medicalinstruments. Applications include tracking of instruments such asultrasound probes, biopsy needles, ablation instruments, and so on.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and description below. Other features,objects, and advantages of the invention will be apparent from thefollowing description of the preferred embodiments, the drawings, andthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the figures, wherein like numerals represent like partsthroughout the several views:

FIG. 1 illustrates a schematic of a magnetic transmitter and sensorintegrated with an ultrasound imaging system;

FIG. 2 illustrates a schematic of prior art magnetic sensor cable andsignal conditioning elements;

FIG. 3 illustrates a schematic of the magnetic sensor cable withimproved signal conditioning elements;

FIG. 4 illustrates states and transitions of a single-channel DC pulsedmagnetic sensor signal with and without the improved signal conditioningelements;

FIG. 5 illustrates extension of the single-channel DC pulsed magneticsensor signal to a three channel magnetic sensor signal stream withpolarity switch timing;

FIG. 6 illustrates application of the improved signal conditioningschema to a single-channel AC driven magnetic sensor signal withpolarity switch timing.

DETAILED DESCRIPTION OF THE INVENTION

The ideal magnetic tracking system receives 100% of its signal inputexclusively from the sensor coil, where the sensor signal is a responseto a transmitted time varying magnetic field. The sensor coil signaltraverses the sensor assembly interconnect system travelling from thesensor coil through cable wires, to and through the connector, andthrough signal conditioning such as an amplifier and analog-to-digitalconverter mounted on a printed circuit board. The interconnect systemcomponents generate spurious signals in response to the transmitted timevarying magnetic field. These spurious signals sum to corrupt theotherwise ideal sensor coil signal, and thus induce position andorientation error of the tracked instrument.

The invention described herein electronically periodically switchespolarity of the summed spurious signal, enabling its self-cancellation.The invented polarity switch method and apparatus is applied to removethe spurious error-inducing signals generated within the interconnect,leaving the desired sensor coil signal uncorrupted.

Referring to FIG. 1, a magnetic position tracking system 100 includes amagnetic sensor 1, a magnetic transmitter 4, a computer 7 and aninstrument 2 whose position is being tracked. Magnetic sensor 1 isconnected to the computer 7 via cable 5 and connector 6. Magnetictransmitter 4 is connected to computer 7 via cable 50. Magnetic sensor 1outputs signals in response to the time derivative of magnetic fields,dB/dt, generated by the magnetic transmitter 4. Computer 7 receives theoutput signals from the magnetic sensor 1 by way of cable 5 andconnector 6 and computes the position of magnetic sensor 1 relative tothe magnetic transmitter 4.

Magnetic sensor 1 may contain one or more signal channels. In oneexample, a typical 6 degree of freedom magnetic position tracking systemmay be constructed using 3 signal channels within magnetic sensor 1combined with 3 orthogonal magnetic transmitting coils housed withintransmitter 4. For better clarity in this description, a single signalchannel is described, because the operation of any additional signalchannel is identical.

Referring to FIG. 2, a single signal channel magnetic position trackingsystem 110, includes a magnetic sensor coil 13, a connector 6, anamplifier 8, an analog to digital (A/D) converter 9 and a processor 10.Coil 13 is connected to the amplifier 8 via a pair of twisted wires 5and via connector 6. The sensing signal passes through the amplifier 8,then through the A/D converter 9 and then goes to processor 10.

Coil 13 detects the time derivative of the magnetic field, dB/dt,generated by the transmitter 4 according to the formula

${EMF}_{coil} = {A*N*U*\frac{\mathbb{d}B}{\mathbb{d}t}}$

-   A=area of coil 13 in square meters-   N=number of turns in coil 13-   U=free space permeability-   dB/dt=time rate of change of the magnetic flux density, B, from    transmitter 4, in Tesla per second.

It is important to ensure that coil 13 is the only element of magneticsensor 1 that is responsive to the magnetic signal from transmitter 4.Any additional signal sources between coil 13 and A/D converter 9 willresult in an incorrect position computation for sensor 1. Prior artsystems depend upon a high quality twisted pair cable 5 to conduct theEMF from coil 13 to connector 6. The twisted pair cable 5 providescancellation of magnetic signals by way of forming small opposing loopsalong its length, causing the EMF of each successive loop to changepolarity with respect to its neighbors and thereby to cancel the effectsof any external magnetic fields. This cancellation works well in auniform magnetic field. However, in a gradient magnetic field, the dB/dtmagnitude is not uniform along cable 5 and therefore the EMF forsuccessive loops is not uniform. In this case cable 5 introduces a cableerror, EMF_(cable). EMF_(cable) has the highest magnitude when cable 5is placed on or near the transmitter 4, due to the high gradient fieldnear the transmitter 4. An example of this occurrence is when instrument2 is an ultrasound transducer and the operator inadvertently pulls cable5 across the transmitter 4.

An additional source of error occurs where the signals from coil 13 passthrough connector 6. In most high density pin type connectors, the pinsform a parallel path over their mating length. This path has a net areadescribed by the product of pin length and pin separation. This net areais shown as a connector pin loop 14 in FIG. 2. The EMF from connectorpin loop 14 is then described as:

${EMF}_{connector} = {L_{p\; i\; n}*W_{p\; i\; n}*U*\frac{\mathbb{d}B}{\mathbb{d}t}}$

-   L_(pin)=length of a connector pin-   W_(pin)=pin separation distance-   U=free space permeability-   dB/dt=time rate of change of the magnetic flux density, B, from    transmitter 4

An important factor with the EMF error from loop 14 is that loop 14 maybe located near transmitter 4 while sensor 1 may be near the outsidelimits of its range. Thus dB/dt at loop 14 may be orders of magnitudelarger than the dB/dt at coil 13. This could occur, for example, if anultrasound operator positions computer 7 and connector 6 near thetransmitter 4 due to space constraints in a procedure room. Prior artsystems commonly place a restriction on the position of the connector 6relative to the transmitter 4, a common restriction being 0.6 meters ofminimum separation. Prior art systems also commonly employ a magneticshield around connector 6, to decrease the dB/dt magnitude at loop 14.Such a shield adds cost and bulk to connector 6, and can causedistortion of the magnetic field transmitted by transmitter 4 if placedtoo closely.

An additional source of EMF error is the net loop area of the printedcircuit board traces, as the physical paths of the signal lines throughamplifier 8 and into A/D 9 are separate. The loop formed by theseprinted circuit board traces is shown by trace area 15 in FIG. 2. Tracearea 15 error is important because circuitry used to energizetransmitter 4 is contained within computer 7 and there is commonly someleakage dB/dt from this circuitry. Since it is desirable to fit computer7 into a small form factor, the spacing between this energizingcircuitry and trace area 15 may be only a few tens of millimeters. Thiscan result in a significant leakage dB/dt component being present attrace area 15, giving:

${EMF}_{trace} = {A_{trace}*U*\frac{\mathbb{d}B}{\mathbb{d}t}}$

-   A_(trace)=trace loop area-   U=free space permeability-   dB/dt=time rate of change of the magnetic flux density, B, from    transmitter 4

Prior art systems protect area 15 using magnetic shielding and alsoattempt to locate the transmitter drive circuitry as far from area 15 asis practical.

Once the signal from coil 13 is digitized by the A/D converter 9 it isno longer susceptible to dB/dt effects from transmitter 4 and isprocessed by processor 10.

The total signal at the input of the A/D converter 9 is thus:EMF _(total) =EMF _(coil) +EMF _(cable) +EMF _(connector) +EMF _(trace)

The last three terms of this equation are significant errors that needto be minimized.

The above mentioned cable, connector and trace errors (EMF_(coil),EMF_(connector), EMF_(trace)) are minimized in the present invention byperiodically switching the polarity of the noise signal. Referring toFIG. 3, in one embodiment of the present invention, a single signalchannel magnetic position tracking system 120, includes a magneticsensor coil 13, a connector 6, a dual single-pole-double-throw (SPDT)analog switch 18, an amplifier 8, an analog to digital (A/D) converter9, a polarity control 11, a multiplier 12 and a processor 10. Coil 13 isconnected to the amplifier 8 via a pair of twisted wires 5 and viaconnector 6. The sensing signal passes through the SPDT analog switch18, the amplifier 8, then through the A/D converter 9, then through themultiplier 12 and then goes to processor 10. Multiplier 12 also receivesinformation from the polarity control 11. Polarity control 11 controlsthe polarity of the sensor signal at the end of the coil terminals.Polarity control 11 is set to output a logic 0 or a logic 1. Logic 0 isinterpreted by multiplier 12 and switch 14 as normal or non-invertingpolarity (value=1) and logic 1 is interpreted as inverted polarity(value=−1). The effect of switch 18 and multiplier 12 is to negate thepolarity of coil 13 as seen by the A/D converter 9, and tosimultaneously negate the data from the A/D converter 9 as seen byprocessor 10. The net effect is that the signal from coil 13 as seen byprocessor 10 does not change sign regardless of the state of polaritycontrol 11. The error inputs, EMF_(cable), EMF_(connector), andEMF_(trace), however, change polarity at processor 10 in accordance withthe state of polarity control 11.

Referring to FIG. 4, the state of polarity control 11 is synchronizedwith the operation of transmitter 4 so that it is logic 0(non-inverting) during the first pulse A 19 and logic 1 (inverting)during the second pulse B 20. EMF_(total) for the rising and fallingedges of pulse 19 are integrated within processor 10 to produce anoutput proportional toEMF_(coil)+EMF_(cable)+EMF_(connector)+EMF_(trace)

This equation is described in U.S. Pat. No. 6,172,499, the contents ofwhich are expressly incorporated herein by reference. At the boundarybetween pulse 19 and pulse 20, polarity control 11 is switched to logic1 and multiplier 12 is set to negate data from A/D 9. The EMF_(total)for the rising and falling edges of pulse B is integrated withinprocessor 10 to produce an output proportional toEMF_(coil)−EMF_(cable)−EMF_(connector)−EMF_(trace)

If we add the integral results from first pulse 19 and second pulse 20and divide by two, the resulting average is an integral proportionalonly to EMF_(coil). Since the positions of computer 7, connector 6, andportions of cable 5 are relatively stable with respect to transmitter 4,the magnitudes of EMF_(cable), EMF_(connector), and EMF_(trace) remainessentially constant during the pulse AB sequence. The present inventionthus eliminates the need to shield loop 14, area 15, and eliminatesgradient error from cable 5.

Placing a lowpass filter at the output of multiplier 12 can alsoaccomplish the averaging function of the first pulse 19 and second pulse20 sequence. The lowpass filter should be chosen such that the ripple atthe output of multiplier 12 as an amplitude function ofEMF_(cable)+EMF_(connector)+EMF_(trace)

is within acceptable limits and the system response bandwidth isadequately fast. For example, in a system employing the presentinvention, a 4th order infinite impulse response (IIR) filter,implemented in a digital signal processor (DSP), with a cutoff frequencyof 2 Hz is adequate for a system employing a three axis transmitter 4and a three axis sensor 1 operating at 240 transmitter pulses persecond.

In addition to magnetic EMF error cancellation, the present inventionmay also be employed to remove EMF errors from sources such as groundcoupling. Current from computer 7 flowing into transmitter 4 may inducesome resistive voltage drops within the conductors of computer 7. Oneimportant conductor is the grounding system. Generally the circuitrywill employ a ground plane on a printed circuit board. This ground planegenerally has a small but measurable resistance, on the order of amilliohm for points a few centimeters apart. Imperfections in amplifier8, ground feedthrough from biasing circuitry, and numerous otherparasitic sources can cause error signals to appear at the output ofamplifier 8. Collectively these EMF error sources are shown as circuiterror source 17. Source 17 will exhibit a reasonably constant responseto each of pulse 19 and pulse 20 in FIG. 4. Due to the constant natureof this response, the multiplier 12 and polarity control 11 will causethe error from source 17 to be periodically inverted. The error source17 is thus removable by averaging or lowpass filtering as previouslydescribed.

Referring to FIG. 5, in another embodiment of the present invention,pulse 19 and pulse 20 are each comprised of multiple pulses. In thisexample, transmitter 4 is comprised of 3 orthogonal coils, referred toas X, Y, and Z respectively, energized sequentially. X axis pulse 21represents the X coil excitation, Y axis pulse 22 represents the Y coilexcitation, and Z axis pulse 23 represents the Z coil excitation. Thecombination of pulses 21, 22, and 23 herein referred to firsttransmitter sequence 24 and second transmitter sequence 25. Using thedevice described in U.S. Pat. No. 6,172,499, as an example, the responseof sensor 1 to each of the pulses 21, 22, 23 in first sequence 24 isprocessed in the same manner as previously disclosed for first pulse 19and stored. Next, polarity control 11 is switched and the response ofsensor 1 to each of the pulses 21, 22, 23 in second sequence 25 iscomputed and averaged with the corresponding response values from firstsequence 24. The sequence of FIG. 5 is useful because analog switch 18may have some undesirable parasitic error effects on the output of coil13. One such effect is commonly known as charge injection. The injectioncomponents change amplitude and polarity synchronously with polaritycontrol 11 and thus appear as a transient offset at the output ofmultiplier 12. Introducing a short amount of dead time 26 between thefirst sequence 24 and the second sequence 25 will allow this transientoffset to decay to zero before being sampled by processor 10.

The system of FIG. 3, may also be use for error reduction in an ACmagnetic tracking system. FIG. 6 shows a pictorial description of keywaveforms present at the input of processor 10 when the system 120 ofFIG. 3 is operated to cancel transmitter induced offset signals in an ACmagnetic tracking system. Transmitter 4 emits an AC magnetic field 27.Sensor coil 13 outputs an EMF proportional to the time derivative ofmagnetic field 27 according to the formulaEMF _(coil) =A*N*U*B*sin ωt

-   A=area of coil 13 in square meters-   N=number of turns in coil 13-   U=free space permeability-   B=peak to peak magnitude of field, in Tesla-   ω=angular frequency of magnetic field, in radians per second-   t=time, in seconds

Parasitic, unbalanced loops exposed to the magnetic field fromtransmitter 4 are added to the signal from coil 13 and the digitizedsignal at processor 10 is described asEMF _(total)=(EMF _(coil) +EMF _(cable) +EMF _(connector) +EMF_(trace))*sin ωt

-   EMF_(coil) sin ωt=signal from coil 13 due to magnetic field from    transmitter 4-   EMF_(cable) sin ωt=induced EMF due to gradient field of transmitter    4 acting on cable 5.-   EMF_(connector) sin ωt=induced EMF in connector pin loop 14 due to    magnetic field from transmitter 4.-   EMF_(trace) sin ωt=induced EMF from printed circuit board trace    loops-   EMF_(trace) sin ωt=induced EMF in trace area 15 due to magnetic    field from transmitter 4

Ideally, EMF_(coil) sin ωt would be the only signal digitized by the A/Dconverter 9 and processed by processor 10 and by a demodulator.EMF_(cable) sin ωt, EMF_(connector) sin ωt, and EMF_(trace) sin ωt areundesireable signals.

The total signal at the A/D converter 9 due to transmitter 4 isdescribed asEMF _(total)=(EMF _(coil) +EMF _(cable) +EMF _(connector) +EMF_(trace))*sin ωt

After demodulation and detection in processor 10, the valuecorresponding to EMF_(total) is stored and the polarity control 11 isswitched. The output of the A/D converter 9 is then equalEMF _(total)=(EMF _(coil) −EMF _(cable) −EMF _(connector) −EMF_(trace))*sin ωt

Demodulating and detecting this second sequence and averaging with thestored result from the first results in an output value proportionalonly to EMF_(coil). It should be noted that it is not required that theAC magnetic field 27 be continuous, nor fixed in frequency. Thetechnique shown will work with time division multiplexed AC magneticfields, and with fixed, variable, or multiple frequencies.

In the embodiment of FIG. 6, the gain of amplifier 8 was set to unity tosimplify the expressions. The waveforms are shown in continuous timeformat for clarity purposes, although in actuality the waveforms shownin FIG. 6 are discrete digital values output by the A/D converter 9.FIG. 6, assumes that the sampling rate of the A/D converter 9 is highenough to accurately capture the details shown.

The embodiment of FIG. 3 may be employed on numerous other signaltransmission methods used in magnetic tracker art by employing thefollowing principals:

-   1) Define a measurement sequence, including magnetic transmitter    excitations and receipt of magnetic signals from sensor coils.-   2) Feeding coil signals into a switching array capable of reversing    the coil polarity relative to subsequent interconnect and processing    elements. The switching array should be located such that parasitic    loops are located between the switching array and the A/D converter.-   3) Controlling the switching array such that the processor receiving    A/D data inverts the data synchronously with coil polarity changes    at the output of the switching array.-   4) Alternating the polarity of the switching array and A/D sign    inversion such that these operations are synchronous with the    defined magnetic transmitter excitation sequences.-   5) Averaging alternate sign inverted processed data sequences such    that the offset components cancel, or alternatively low pass    filtering the processed data sequence, or alternatively storing a    sequence of a first polarity, subtracting a sequence of opposing    polarity, and utilizing the remainder offset value to correct future    readings.

Several embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A system for magnetic position tracking of adevice comprising: a magnetic transmitter including at least onetransmitter coil, wherein said transmitter coil outputs a transmittedmagnetic field including a time derivative component; a magnetic sensorincluding at least one sensor coil, wherein said sensor coil includescoil terminals having a polarity, and wherein said sensor coil isresponsive to said time derivative component of said transmittedmagnetic field and outputs a sensor signal; a computing system forcomputing position and angular orientation data of a device based onsaid sensor signal; and a polarity inverter configured to connect tosaid coil terminals and to cause the polarity of the coil terminals tobe reversed according to a switching signal, wherein magnetic fielderrors that are caused by connectors and traces that are responsive tosaid time derivative component of said transmitted magnetic field aresubstantially removed from the sensor signal.
 2. The system of claim 1,further comprising a sign inverter configured to invert a digitizedoutput from an analog to digital (A/D) converter, and wherein said signinverter is operated concurrently with said switching signal, so thatsaid polarity of said coil terminals is maintained at said computingsystem's input.
 3. The system of claim 2, further comprising asynchronizer configured to operate concurrently with said magnetictransmitter.
 4. The system of claim 2, further comprising averagingmeans.
 5. The system of claim 2, where said sign inverter is alsoconfigured to invert the sensor signal at said (A/D) converter's input.6. The system of claim 1, wherein said transmitted magnetic fieldcomprises a sinusoid.
 7. The system of claim 6, wherein said sinusoidcomprises a plurality of sine waves.
 8. The system of claim 6, whereinsaid sinusoid is continuous with respect to time.
 9. The system of claim6, wherein said sinusoid is time division multiplexed.
 10. The system ofclaim 1 wherein said transmitted magnetic field comprises one oftrapezoidal, triangular, half sinusoid, exponential, or square amplitudeversus time characteristics shape.
 11. The system of claim 1, whereinsaid polarity inverter is located adjacent to said magnetic sensor. 12.The system of claim 1, wherein said polarity inverter is connected tosaid magnetic sensor via a twisted pair cable.
 13. The system of claim1, wherein said polarity inverter comprises an analog switch.
 14. Thesystem of claim 1, wherein said switching signal is transmittedwirelessly.
 15. The system of claim 4, wherein said averaging means isconfigured to sum signals received with opposite polarity from said signinverter.
 16. The system of claim 4, wherein said averaging meanscomprises a lowpass filter.
 17. A method for magnetic position trackingof a device comprising: providing a magnetic transmitter including atleast one transmitter coil, wherein said transmitter coil outputs atransmitted magnetic field including a time derivative component;providing a magnetic sensor including at least one sensor coil, whereinsaid sensor coil includes coil terminals having a polarity, and whereinsaid sensor coil is responsive to said time derivative component of saidtransmitted magnetic field and outputs a sensor signal; providing acomputing system for computing position and angular orientation data ofa device based on said sensor signal; providing a polarity inverterconfigured to connect to said coil terminals and to cause the polarityof the coil terminals to be reversed according to a switching signal;substantially removing, from the sensor signal, magnetic field errorsthat are caused by connectors and traces that are responsive to saidtime derivative component of said transmitted magnetic field.
 18. Themethod of claim 17, further comprising providing a sign inverterconfigured to invert a digitized output from an analog to digital (A/D)converter, and wherein said sign inverter is operated concurrently withsaid switching signal, so that said polarity of said coil terminals ismaintained at said computing system's input.
 19. The method of claim 18,further comprising providing a synchronizer configured to operateconcurrently with said magnetic transmitter.
 20. The method of claim 18,further comprising providing averaging means.
 21. The method of claim18, where said sign inverter is also configured to invert the sensorsignal at said (A/D) converter's input.
 22. The method of claim 17,wherein said transmitted magnetic field comprises a sinusoid.
 23. Themethod of claim 22, wherein said sinusoid comprises a plurality of sinewaves.
 24. The method of claim 22, wherein said sinusoid is continuouswith respect to time.
 25. The method of claim 22, wherein said sinusoidis time division multiplexed.
 26. The method of claim 17 wherein saidtransmitted magnetic field comprises one of trapezoidal, triangular,half sinusoid, exponential, or square amplitude versus timecharacteristics shape.
 27. The method of claim 17, wherein said polarityinverter is located adjacent to said magnetic sensor.
 28. The method ofclaim 17, wherein said polarity inverter is connected to said magneticsensor via a twisted pair cable.
 29. The method of claim 17, whereinsaid polarity inverter comprises an analog switch.
 30. The method ofclaim 17, wherein said switching signal is transmitted wirelessly. 31.The method of claim 20, wherein said averaging means is configured tosum signals received with opposite polarity from said sign inverter. 32.The method of claim 20, wherein said averaging means comprises a lowpassfilter.
 33. A method comprising: outputting, by a transmitter coil, atransmitted magnetic field including a time derivative component;outputting, by a sensor coil that is responsive to the time derivativecomponent of the transmitted magnetic field, a sensor signal; computingposition and angular orientation data of the sensor coil based on thesensor signal; reversing a polarity of coil terminals of the sensor coilaccording to a switching signal; and substantially removing, from thesensor signal, magnetic field effects caused by conductive connectionsthat are responsive to the time derivative component of the transmittedmagnetic field.